fix(inputs.conntrack): Skip gather tests if conntrack module not loaded

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

This PR skips the inputs.conntrack tests for machines where the conntrack kernel module is not loaded. In those cases tests currently fail with

plugins/inputs/conntrack# go test -v -race
=== RUN   TestNoFilesFound
--- PASS: TestNoFilesFound (0.00s)
=== RUN   TestDefaultsUsed
--- PASS: TestDefaultsUsed (0.00s)
=== RUN   TestConfigsUsed
--- PASS: TestConfigsUsed (0.00s)
=== RUN   TestCollectStats
    conntrack_test.go:131: 
                Error Trace:    /home/sven/Development/InfluxData/telegraf/plugins/inputs/conntrack/conntrack_test.go:131
                Error:          Received unexpected error:
                                Conntrack input failed to collect metrics. Is the conntrack kernel module loaded?
                Test:           TestCollectStats
    panic.go:540: PASS: NetConntrack(bool)
--- FAIL: TestCollectStats (0.00s)
=== RUN   TestCollectStatsPerCpu
    conntrack_test.go:217: 
                Error Trace:    /home/sven/Development/InfluxData/telegraf/plugins/inputs/conntrack/conntrack_test.go:217
                Error:          Received unexpected error:
                                Conntrack input failed to collect metrics. Is the conntrack kernel module loaded?
                Test:           TestCollectStatsPerCpu
    panic.go:540: PASS: NetConntrack(bool)
--- FAIL: TestCollectStatsPerCpu (0.00s)
FAIL
exit status 1
FAIL    github.com/influxdata/telegraf/plugins/inputs/conntrack 0.017sl

[telegraf-tiger]

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DEB	RPM	TAR GZ	ZIP
amd64.deb	aarch64.rpm	darwin_amd64.tar.gz	windows_amd64.zip
arm64.deb	armel.rpm	darwin_arm64.tar.gz	windows_i386.zip
armel.deb	armv6hl.rpm	freebsd_amd64.tar.gz
armhf.deb	i386.rpm	freebsd_armv7.tar.gz
i386.deb	ppc64le.rpm	freebsd_i386.tar.gz
mips.deb	riscv64.rpm	linux_amd64.tar.gz
mipsel.deb	s390x.rpm	linux_arm64.tar.gz
ppc64el.deb	x86_64.rpm	linux_armel.tar.gz
riscv64.deb		linux_armhf.tar.gz
s390x.deb		linux_i386.tar.gz
		linux_mips.tar.gz
		linux_mipsel.tar.gz
		linux_ppc64le.tar.gz
		linux_riscv64.tar.gz
		linux_s390x.tar.gz
		static_linux_amd64.tar.gz